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Biological control of winter annual grass weeds in winter wheat

Phillip W. Stahlman and Pamela A. Harris
Fort Hays Branch, KS Agric. Experiment Station
1992 Wheat Technology Conference

Objective: Native bacteria are being isolated from Kansas soils and the roots of several crop and weed species and evaluated for biological control annual brome species (cheat, downy brome, Japanese brome) and jointed goatgrass in winter wheat.

Justification: The widespread adoption of conservation tillage systems in semi-arid environments and a shift to less competitive, semidwarf wheat cultivars have been accompanied by a dramatic increase and spread of winter annual grass weeds in winter wheat. Downy brome and other Bromus species infest and estimated 14 million acres of winter wheat in the U. S. Chemical herbicides currently registered for control of winter annual bromes in wheat perform inconsistently, lack selectivity or are prohibitively expensive. Jointed goatgrass is an especially troublesome grass weed because it is genetically related to and highly competitive with wheat, and effective herbicidal control is lacing. Jointed goatgrass infests an estimated 3 million acres of wheat in 10 Great Plains and western states and is spreading rapidly.

Increased awareness of possible environmental and health risks associated with the intensive use of synthetic herbicides and increased soil erosion with tillage methods for weed control has renewed interest in alternative methods of weed control. Biological weed control can reduce the need for both synthetic chemicals and tillage, which should lower crop production costs and enhance environmental quality.

Preliminary Results: Over 800 isolates have been screened in laboratory bioassays. Initial screenings have identified 180 isolates which meet the criteria of inhibition of downy brome root elongation without affecting winter wheat, whereas 202 isolates inhibit Japanese brome root growth. In addition, 129 isolates inhibit root elongation of jointed goatgrass. All promising isolates are currently being assessed in soil in growth chamber studies. Of the 38 isolates already screened in soil, none isolates still demonstrate inhibitory effects on downy brome shoot and/or root growth, and three isolates exhibit similar results with jointed goatgrass. A field study with ten isolates was established in the fall of 1990 and harvested in June 1991. Although adverse environmental conditions (high temperatures/low moisture) were present at whet planting, wheat yields wee 13% greater with the addition of one isolate. Additional field studies have been established in the fall of 1991.

Current Status of Research Area: Currently, USDA personnel at Washington State University have the only other public research program emphasizing biological control of winter annual grass weeds in winter wheat. While the two programs are similar, the endeavors are independent and investigate several different aspects of biological control. It is important to investigate the efficacy of bacterial isolates obtained in the central Great Plains. Since these microorganisms exist under more adverse environmental conditions, the bacteria may have stronger survival and proliferation profiles. Also, strong cooperative atmosphere enables both programs to access and benefit from the research at both locations. We are participating in a USDA Pilot Program to field test a promising isolate from Washington in five states, including Kansas. A reciprocal program is anticipated after Kansas isolates are field tested within the state.

Additional Research Fronts:

A. Several application methods will be tested to determine the best method to ensure bacterial survival and maximum inhibitory performance. Several methods may be tested, including:

  1. Seed inoculation – wheat seed will be inoculated with inhibitory bacteria before planting. The inoculant may be similar to Rhizobium inoculants used with many legumes. Bacteria are absorbed/adsorbed to carrier material (peat, clay, etc.) and then applied to the seed using a sticker (water, gum arabic, etc.).
  2. Furrow inoculation – inoculant will be made with a carrier material as described above. Instead of applying the inoculant directly to the seed, the material will be placed in the seed furrow at planting.
  3. Tank mixtures – the inhibitory bacteria will be tank mixed with common wheat herbicides and applied as a liquid with a sprayer.

B. Promising isolates will be evaluated with commercial herbicides now available for annual grass weed control in winter wheat to examine potential enhancement of herbicide effectiveness. We believe that a probable use for inhibitory bacteria will be herbicide enhancement, thus possibly reducing the quantity of herbicide required for effective weed control.

C. All isolates will eventually be screened against the weed species previously mentioned in laboratory and growth chamber bioassays. Also, promising inhibitory isolates will be tested against other common wheat varieties and other crops normally grown in rotation with winter wheat or in adjacent areas to ensure the bacteria are not inhibitory to these crops.

D. Field studies of inhibitory isolates will examine the efficacy of the bacteria under field conditions. The bacteria must be able to establish and proliferate after initial inoculation to facilitate colonization of weed roots. However, the bacteria should only be present in high numbers during the period weed inhibition, after which they should die off to undetectable levels, thus lowering their impact on the normal soil fora.

Bacteria which selectively control or reduce the competitiveness of these annual grass weeds are a promising alternative weed control method. Microbial weed control agents are unlikely to replace synthetic herbicides, but they can complement current weed management systems. Benefits of adopting this new weed management technology include increased crop production efficiency, expanded use of conservation tillage systems, decreased dependence on synthetic herbicides, and reduced risk of surface and groundwater contamination. Additionally, the methodologies developed from this research could potentially be used to develop microbial weed control agents for any weed in any cropping system.